Regulation by hypoxia of methionine adenosyltransferase activity and gene expression in rat hepatocytes.
OBJECTIVE
Oxygen supply to the hepatic parenchyma is compromised by long- or short-term ethanol consumption and pathological conditions such as cirrhosis. Impairment in the production of S-adenosyl-L-methionine, the major methylating agent, occurs during hypoxia. In this study, the molecular mechanisms implicated in the regulation of S-adenosyl-L-methionine synthesis by oxygen levels were investigated.


METHODS
Rat hepatocytes were isolated and cultured under normoxic (21% O2) or hypoxic (3% O2) conditions for different periods. Methionine adenosyltransferase activity, messenger RNA levels, and nuclear transcription were evaluated.


RESULTS
Methionine adenosyltransferase was inactivated in hepatocytes kept under low oxygen levels. Hypoxia induced the expression of nitric oxide (NO) synthase, and the inactivation of methionine adenosyltransferase was prevented by the NO synthase inhibitor N(G)-monomethyl-L-arginine methyl ester. Methionine adenosyltransferase messenger RNA levels were down-regulated by hypoxia, through a mechanism that might involve a hemoprotein. Hypoxia dramatically reduced methionine adenosyltransferase gene transcription, and messenger stability was also decreased, although to a lesser extent.


CONCLUSIONS
We have established the molecular basis for the regulation of methionine adenosyltransferase activity and gene expression by hypoxia. NO-mediated inactivation and transcriptional arrest seem to be the two major pathways by which oxygen levels control hepatic methionine adenosyltransferase, the enzyme necessary for methylation reactions and for the synthesis of polyamines and glutathione.
Avila
M A
MA
Instituto de Investigaciones Biomédicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
Carretero
M V
MV
Rodriguez
E N
EN
Mato
J M
JM
D000818:Animals; D015687:Cell Hypoxia / Q000502:physiology*; D002478:Cells, Cultured; D015971:Gene Expression Regulation, Enzymologic; D005982:Glutathione Transferase / Q000378:metabolism; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008297:Male; D008716:Methionine Adenosyltransferase / Q000235:genetics / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism; D019001:Nitric Oxide Synthase / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012436:S-Adenosylmethionine / Q000378:metabolism
D012333:RNA, Messenger; D009569:Nitric Oxide; D012436:S-Adenosylmethionine; D019001:Nitric Oxide Synthase; D005982:Glutathione Transferase; D008716:Methionine Adenosyltransferase